BIOLOGY & EVOLUTION OF THE BIVALVIA

An International Meeting to Focus Solely on the Bivalvia | 14 -17th Septmeber 1999

Meeting Volume and Abstracts (scroll down)

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ABSTRACTS
There follows, by request, a list of abstracts from the above meeting in alphabetical order.  Thanks are due to Liz Harper and John Taylor, amongst others, for making the meeting such a success.

The Aberrant Jurassic Bivalve Opisoma: How did it Function?
Martin Aberhan

Museum für Naturkunde, Institut für Paläontologie, Invalidenstr. 43,
D-10115 Berlin, Germany.  E-mail:  martin.aberhan@rz.hu-berlin.de

The aberrant bivalve Opisoma is represented in the Lower Jurassic (Middle Toarcian) of northern Chile by O. excavatum Boehm. Its very unusual morphology is characterized by a laterally strongly compressed shell, a very prominent posterior ridge and a very massive, ventrally elongated hinge plate. Lack of modern morphological counterparts for comparison has led to some confusion regarding the orientation of the valves in Opisoma, the position of the ligament and the site of adductor muscle attachment. The excellently preserved Chilean specimens allow to disentangle some of these confusions. What remains open to debate, however, is the opening mechanism.  As Opisoma does not exhibit a clear ligamental area there is a possibility that the ligament has been lost completely in adults. This raises the question whether the valves were opened by muscles. However, the only muscle scar recognized in Opisoma seems to belong to a posterior adductor muscle rather than a "diductor" muscle. Opisoma excavatum is best interpreted as an epibenthic to partly buried species that rested on the anterior area of both the left and the right valve, and maintained its plane of commissure in a vertical position. It became stabilized by weight (extreme shell thickening and large size) and by form (broad triangular cross-section). Apparently, it evolved directly from shallow infaunal ancestors into the niche of edgewise recliners.  Various independent lines of evidence suggest that O. excavatum is a potential candidate for palaeophotosymbiosis. Among these are: large size; thick shell; high skeleton-to-body ratio; adoption of an epifaunal life habit in an otherwise shallow infaunal stock; and palaeoenvironmental considerations. However, the apparent lack of microstructural adaptations to light transmission in a very thick shell is a good reason to be doubtful about a photosymbiotic way of life.
 

The Function of Freshwater Mussels (Bivalvia: Unionoida) in Aquatic Ecosystems

David C. Aldridge

Aquatic Ecology Group, Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, U.K.  E-mail:  d.aldridge@zoo.cam.ac.uk

The international declines and extinctions of freshwater mussels are widely documented and have resulted in the threatened unionoid fauna of many countries being made a conservation priority.  What is less widely appreciated is that mussels perform a number of important roles in freshwater ecosystems which suggest that declines in even the common species could have important knock-on effects to the rest of the biota. For example, suspension feeding by mussel beds can reduce turbidity and modify plankton communities; mussel glochidia larvae are important parasites of some fish; mussels function as obligatory hosts to bitterling fish and Unionicola mites; external surfaces of mussels can be an important site of attachment for sponges, zebra mussels and even barnacles.  This paper reviews these roles and discusses how declines in mussel communities can be controlled.
 

Extinction and Radiation of Bivalves in the Late Devonian

Michael R. W. Amler

Institut für Geologie und Paläontologie der Philipps-Universität Marburg, Abt. Invertebraten-Paläontologie, Hans-Meerwein-Strasse, D-35032 Marburg, Germany.
E-mail:  amler@mailer.uni-marburg.de

The phylogenetic development of the Bivalvia during the Devonian is characterized by a distinctive diversification of many Early Palaeozoic taxa culminating in the Middle Devonian. However, the Kellwasser Crisis near the end of the Early Late Devonian marks a major break in bivalve evolution.   This development is closely linked with the ecological evolution within bivalves corresponding with the relation to their specific habitats and the general diversification of ecological niches until the end of the Frasnian.  The Early and Middle Famennian is generally void of diverse bivalve faunas.  Again, close relationships to ecological and biofacies conditions are observable. Several taxa which had their acme during Early and Middle Devonian time, e.g. Actinodesma, Gosseletia, Pseuda-viculopecten, some pterinopectinids, Paracyclas, Cardiola and Buchiola, decreased dramatically during the Famennian and became extinct at the end of the Devonian. Other taxa of uncertain systematic position and partly unknown life habits disappeared already during the early Famennian, e.g. Praecardium, Opisthocoelus, Prosochasma, Loxopteria, Carydium, Prosocoelus.  But, in contrast to earlier views, a new diversification of bivalves started already with the Late Famennian transgression (“Strunian”). Most of these taxa display “modern”, i.e. Late Palaeozoic, characters, crossed the Devonian/Carboniferous transition and reached a maximum in diversification during the Early Carboniferous. Important members of this radiation phase are pteriomorphs of the genera Aviculopecten, Limipecten, Fasciculiconcha, Streblochondria, Streblopteria, Euchondria, Undopecten, Pernopecten, as well as Prothyris, Edmondia and other members of the Anomalodesmata. In contrast, some other taxa of the pteriomorphs and most palaeotaxodonts display no diagnostic relationships to earlier or later faunas. They evolved from a diverse group of Devonian ancestors and persisted across the D/C boundary with Dinantian descendants, although detailed lineages are unclear, e.g. Leptodesma and Leiopteria among the pterineids, both displaying indistinctive or habitat controlled morphology. The same applies to the Palaeotaxodonts which were unaffected by global or regional changes during that interval and link the Devonian ancestors with their Carboniferous descendants.  In summary, the bivalve fauna of the Latest Devonian (“Strunian”) exhibits a transitional character from the Devonian to the Carboniferous with a successive evolutionary transition across the D/C boundary rather than a sharp faunal break whereas a distinctive extinction event occurred during the Kellwasser Crisis near the Frasnian/Famennian boundary. Consequently, most Late Palaeozoic taxa originated already in the late Devonian rather than after the D/C boundary.
 

Phylogeography of Two Pearl Oysters Pinctada margaritifera and P. mazatlanica using Mitochondrial Markers

Sophie Arnaud(1), F. Bonhomme(1) and F. Blanc(2)

(1) Laboratoire « Génome, Populations, Interactions » Station Méditerranéenne de l’Environnement Littoral, 34200 Sete, France.  E-mail:  S-arnaud@crit.univ-montp2.fr
(2) Laboratoire de Zoogéographie, Route de Mende, 34199 Montpellier Cedex 5, France

We studied the genetic variability of two pearl oysters species, Pinctada margaritifera, which is ranging from Indian Ocean to Central Pacific, and P. mazatlanica which is found on American coasts from North Mexico to North Peru. P. mazatlanica is regarded either as a subspecies of P. margaritifera, or as a distinct species on the basis of morphological criteria.  Allozymic data performed previously showed a close relationship of these two entities and led to the hypothesis that P. mazatlanica results from the colonisation of the American coasts by P. margaritifera cumingi from Polynesia. To test this hypothesis and give a further insight in the taxonomical status of the two taxa, we studied the genetic variation within and among populations of P. mazatlanica from North Mexico to Panama Pacific coasts, and of P. margaritifera cumingi from Cook to Marquesas Islands using the restriction polymorphism of two mitochondrial DNA genes (12S and Cox). A strong global structuring was observed among samples of P. mazatlanica, whereas at the same geographic scale, none or little differentiation was evidenced for P. margaritifera. The lack of common restriction haplotype did not permit however to ascertain the links between the two taxa. Nevertheless, the RFLP characterisation of P. margaritifera from Mauritius in the Indian ocean showed a greater genetic proximity of P. margaritifera and P. mazatlanica from central and eastern Pacific than these are from the Indian ocean subspecies. This would speak in favour of a direct link between the Pacific taxa, but raises the question of the taxonomical status of P. margaritifera at the scale of its whole range.
 

 Naticid Predation on the Shells of Middle Miocene Corbulids - A Comparison (Ipolydamsd, Börzsöny Mountains, Hungary

Dávid Árpád

Kroly Eszterhzy Teachers’ Training College, Department of Geography, Eger, Hungary.
E-mail:  davida@gemini.ektf.hu

Naticid gastropod predation on corbulids is described to be unusual because of low rate of success and lower frequency than predicted by a net energy maximization model. It is attributed to the conchiolin layers within the valves acting as an effective barrier to chemical boring by predatory gastropods. In this study naticid - prey interaction has been examined in the case of two corbulid species - Corbula (Varicorbula) gibba Olivi and Corbula carinata Dujardin.  Borehole site selectivity, prey size selectivity and degree of predation success have been compared.  Boreholes were most frequent on the right valves in the case of both species. The occurrence of incomplete boreholes was more significant on the tests of C. gibba.  The ratio of multiplied borings was higher on the shells of C. gibba. There were no significant differences between the two species regarding site selectivity and prey size selectivity.

Unifying Principles of Particle Processing Mechanisms in Bivalves

Peter G. Beninger

Laboratoire de Biologie Marine, Faculté des Sciences, Université de Nantes, Nantes 44322 Cédex, France.  E-mail:  Peter.Beninger@sut.univ-nantes.fr

Despite the observed diversity of particle processing modes in Bivalves, unifying principles have emerged from new observational techniques and intensive study over the past decade.  Pallial organ anatomy determines the water/particle flow characteristics and processing routes, cilia type determines particle - pallial organ interaction, and mucus type determines the nature of particle processing.   We will focus on effector (cilia and mucus) types used in the processing sequence of capture, transport, selection, ingestion/rejection.  In bivalves possessing laterofrontal cirri (the vast majority), laser confocal observations demonstrate direct cilia-particle interaction at the capture point; transport involves simple cilia and mucus in all bivalves studied, either as classical mucociliary transport or as a modified mucociliary-hydrodynamic transport.  Physical and biochemical fluidization of the mucus-particle strands occurs on the palps in species which use this organ for selection (the vast majority).   Rejection of excess volume or negatively - selected material is universally mucociliary, involving acidic mucopolysaccharides and counter-current transport.  Elevation of rejecta and mucociliary transport above the general mantle epithelium is the rule in all species with gill ventral particle grooves (again, the vast majority).  A specific type of cilium is usually involved.  Ingestion takes place within a fluidized mucus slurry.  Unifying principles continue to be determined, based on the effectors - cilia and mucus - of particle processing in bivalves.

This paper will be given as part of the Feeding Workshop.

Bivalve Gill Abfrontal Ciliation and Mucocyte Types: what they convey about the evolution of this organ

Peter G. Beninger(1) and Suzanne C. Dufour (2)

(1) Laboratoire de Biologie Marine, Faculté des Sciences, Université de Nantes, Nantes Cédex 3, France. E-mail:  Peter.Beninger@sut.univ-nantes.fr
(2) Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0202, U.S.A.

The lack of fundamental data on the abfrontal surface of bivalve gills has prompted a comparative study of cilia and mucocytes on this surface.  These features have been studied by scanning electron microscopy and histology on eight species of bivalves, representing seven families and the four major gill types (Mytilus edulis, Modiolus modiolus, Arca zebra, Placopecten magellanicus, Crassostrea virginica, Spisula solidissima, Mya arenaria and Mercenaria mercenaria).  Inter-species variations were found; gradients in the numbers and diversity of cilia and mucocytes were observed for each gill type. These results indicate that the abfrontal surface had a primitive role in mucociliary cleaning (prior to filament folding), and that the cilia and mucocytes observed in contemporary species are vestigial.  In general, the degree of abfrontal cilia and mucocyte loss parallels the degree of evolution of the gill:  eulamellibranchs have fewer abfrontal cilia and mucocytes than homorhabdic filibranchs. The data are consistent with the interpretation that the loss of the primitive mucociliary cleaning function gave rise to two evolutionary outcomes: (1) selective pressures led to the reduction in numbers and types of abfrontal mucocytes and cilia; and (2) abfrontal mucocytes were retained as they assumed new functions in water flow.
 

 Reproductive Output in Macoma balthica in the Wadden Sea:  do they follow an optimal strategy?

Jan Beukema and Pieter Honkoop

Netherlands Institute for Sea Research, PO Box 59, 1790 Den Burg, Texel, The Netherlands.  E-mail:  jsr@nioz.nl

Annual reproductive output in individual females of the tellinid bivalve Macoma balthica (L.) varies strongly from 0 to about 100,000 eggs.  Expressed as a proportion of the ash-free dry weight (AFDW) of the soft parts, it varies from 0 to about 33% (Honkoop et al. 1999:  “Reproductive investment in the intertidal bivalve Macoma balthica” J. Sea Res. 41, 203-212).  This variation is strongly related to the “condition” of the animals, expressed as the body-mass index (BMI), i.e. AFDW divided by the third power of shell length.  Lean animals with a BMI of 5.6 mg cm-3 or less do not spawn any eggs.  At the other extreme, maximal proportions of about 30% of AFDW are reached at BMI values > 10 mg-3.  Long-term monitoring (> 20 y) of Macoma densities at Balgzand (a tidal-flat area in the westernmost part of the Wadden Sea) revealed that the proportion surviving to the next year was lower at low than at high BMI at the start of the spawning season (ranging from about 0.25 at BMI=5.6 to about 0.65 at first spawning, viz. both as an immediate response (determining the proportion of the weight spawned as eggs) and as a delayed response in following years (via BMI-dependent proportions of the animals spawning also in subsequent years).  Therefore, a trade-off situation exists between the present and the future spawning occasions.  Any further reduction of BMI by a larger immediate spawning would result in lower survival and thus lower outputs in future years.  It may be expected that animals will optimize total lifetime reproductive output.  Lifetime reproductive outputs were calculated at different strategies on the first spawning occasion, ranging from postponement to the next year (0% output at all BMI values) to maximal output (all mass above BMI=5.6 or 30% of AFDW at all BMI>8.0).  It is concluded that the realized strategy of an increase of reproductive output with “condition” is close to an optimal one.
 

 Marine Bivalves of the Florida Keys: discovered biodiversity

Rüdiger Bieler (1) and Paula M. Mikkelsen (2)

(1)  Department of Zoology, Field Museum of Natural History, Roosevelt Road at
Lake Shore Drive, Chicago, Illinois 60605-2496, U. S. A.  E-mail:  bieler@fmnh.org
(2)  Department of Invertebrates, American Museum of Natural History, Central
Park West at 79th Street, New York, New York 10024-5192, U. S. A.
E-mail:  mikkel@amnh.org

The Florida Keys island group at the southernmost tip of the continental U.S. supports a remarkably diverse marine malacofauna. Surprisingly, after a century of popular and professional shell collecting, the molluscs have never been comprehensively assessed.  Although best known for its coral reefs, the Keys comprise about 10,000 km2 of marine habitat, and include hypersaline ponds, mangrove thickets, seagrass meadows, muddy tidal channels, sandbars, and deep sand plains. This molluscan survey (in part addressing the needs prompted by establishment of the Florida Keys National Marine Sanctuary in 1991) compiled from over 200 original collections, 4,000 museum lots, and 3,000 literature records, revealed over 1,300 molluscan species, including more than 300 bivalves.  These represent a wide taxonomic diversity -50% of recognized families and 70% of superfamilies.  Systematic scrutiny has shown several cryptic species pairs, commonly known under a single taxonomic name, but morphologically different and associated with different habitats (e.g., estuarine Florida Bay versus oceanic coral reefs).  Community analyses show roughly equal proportions of infaunal and epifaunal species, with the latter including “coral reef-important” borers and cementers. Within-Keys distributions include one-third of species ranging the full length of the island chain, one-third so far recorded from a single zone (Upper, Middle, Lower, Tortugas), and one-third overlapping two or more zones.  Species ranges show ca. 50% of Keys bivalves considered “wide ranging” both north and south, but 85% of the remainder decidedly tropical in distribution.  Historical records indicate little species turnover, although habitat shifts from natural to artificial substrata are evident.
 

 On Becoming Sessile: Evolutionary Relationships among the Genera in the Cemented Freshwater Bivalve Family Etheriidae (Bivalvia: Unionoida)

Arthur E. Bogan (1) and Walter R. Hoeh (2)

(1) North Carolina State Museum of Natural Sciences, P.O. Box 29555, Raleigh, NC
27626, U.S.A.  E-mail:  Arthur_Bogan@mail.ENR.STATE.NC.US
(2) Department of Biological Sciences, Kent State University, Kent, OH 44242, U.S.A.

The family Etheriidae (freshwater oysters) has been recognized as a distinct taxon for well over 160 years.  The relationships of this family to other unionoid families and its constituent genera have been debated.  Many malacologists recognize three genera in the Etheriidae: Acostaea (Columbia, South America), Pseudomulleria (India), and Etheria (Africa and Madagascar). Mansur and da Silva (1990) have recently supported this monophyletic view of the Etheriidae. However, Starobogatov (1970) placed the three genera into distinct families: Acostaea in the Mulleriidae, Mullerioidea along with the Mycetopodidae, Etheria remained in Etheriidae, and Pseudomulleria in Pseudomulleriidae, both placed in the Etherioidea.  Similarly, Bonetto (1997) has placed Acostaea in the Acostaeinae in the Mycetopodidae, Etheria in the Etheriinae and Pseudomulleria in the Pseudomulleriinae, both in the Mutelidae.  Thus, the works of Starobogatov (1970) and Bonetto  (1997) contradict the monophyly of the Etheriidae by suggesting instead that the Etheriidae is a polyphyletic assemblage.  These conflicting views on the evolutionary relationships surrounding the etheriid genera hinder the development of a basic understanding of the circumstances involved in the evolution of the sessile habit in freshwater bivalves.  A fundamental question is: did the sessile habit in unionoids evolve once or multiple times?  A monophyletic Etheriidae would support the former hypothesis while a polyphyletic Etheriidae would support the latter.  To evaluate these possibilities, we have conducted phylogenetic analyses of mitochondrial DNA sequences (COI) to examine the relationships of the 3 etheriid genera to representatives of 27 other unionoid genera.  Preliminary analyses firmly place Acostaea within a clade of Anodontites species, currently in the Mycetopodidae.
 

 Mitochondrial and Nuclear DNA Phylogeography of Two Cupped Oysters Crassostrea gigas and Crassostrea angulata

Pierre Boudry and Arnaud Huvet

IFREMER, Station de La Tremblade, Ronce les Bains, BP 133, 17390 La Tremblade, France.  E-mail:  pboudry:ifremer.fr

The taxonomic status of Crassostrea angulata and Crassostrea gigas has long been a matter of controversy. Morphological and physiological similarities, as well as homogeneity in allelic frequencies on allozymes between the populations of the two taxa, lead most authors to suggest to regroup of the two within the same species. European and Asian populations of C. gigas and C. angulata have been studied using microsatellite and mitochondrial DNA markers. The analysis of genetic distances and the distribution of allelic and haplotype frequencies revealed a differentiation between the populations of C. gigas and C. angulata. The data allowed the construction of Neighbor-joining trees for each of the two types of markers. Similar topologies appeared with data on both genomes showing two clusters, but mitochondrial DNA presented much higher genetic differentiation among taxa than microsatellites. The first cluster included the French and Japanese populations and the second the Taiwanese and Portuguese populations. The Asiatic origin of Crassostrea angulata taxa is therefore confirmed. Despite their history, European populations of C. angulata did not show any significant reduction of variability compared to Asian populations.
 

The Effect of Reproduction on Locomotor Performance and Muscle Metabolic Capacities in the Scallop Chlamys islandica

Katherina Brokordt, John Himmelman and Helga Guderley

Dépt. de Biologie, Université Laval, Québec, G1K 7P4 Canada.
E-mail:  Katherina.Brokordt@girog.ulaval.ca

In scallops, during gametogenesis biochemical reserves such as glycogen and proteins are mobilized from the adductor muscle towards the gonad. This mobilization of material is likely to diminish the metabolic capacities of the adductor muscle and thereby the scallops’ escape response. Scallops must make a trade-off between a loss in their capacity to escape from predators and the availability of materials for gametogenesis. We examined the escape response and the recuperation from exhausting exercise in adult scallops Chlamys islandica sampled at different reproductive stages (immature, mature before and after spawning).   In parallel, we measured muscle glycogen, protein and phosphoarginine content, as well as the levels of enzymes that participate during muscle contraction and recovery, such as glycogen phosphorylase (GP), phosphofructokinase (PFK), pyruvate kinase (PK), octopine deshydrogenase (ODH), arginine kinase (AK), and citrate synthase (CS). We also measured the oxidative capacity of mitochondria isolated from the adductor muscle. Immature animals recovered their initial swimming capacity within 6 h, but mature and spawned scallops needed 12 an 18 h respectively. The number of claps (24-26) as well as phosphoarginine and AK levels were similar during the different reproductive stages. However, mature and spawned animals showed a decrease of GP, PFK, PK, ODH and CS levels and a deterioration of oxidative capacity of muscle mitochondria as well as a marked decrease of glycogen contents. Therefore, during gonadal maturation and spawning, C. islandica did not change its clapping capacity, but decreased its glycolytic and aerobic recuperation after an exhausting burst exercise, most likely due to the decreased metabolic capacity of the adductor muscle.
 

Reproduction of the Hermaphroditic Brooding Clam Corbiculina australis in New South Wales: a light and electron microscope study

Maria Byrne(1), Harriette Phelps(2), Tony Church(3) and Jaimie Potts(3)

(1) Department of Anatomy and Histology F13, University of Sydney, NSW 2006, Australia.  E-mail:  mbyrne@anatomy.usyd.edu.au
(2) Department of Biological and Environmental Sciences, University of the District of Columbia, 4200 Connecticut Avenue, Washington, D.C. 20008, U.S.A.
(3) NSW EPA, Locked Bag 1502, Bankstown, NSW 2200, Australia

The freshwater clam Corbiculina australis is an important component of the macrobiota of the river systems of southeastern Australia.  Reproduction of two populations of this clam in the Nepean River, NSW was investigated to document their gametogenic cycle, larval morphology and to determine when they incubate embryos in their gill marsupia.  C. australis is a simultaneous hermaphrodite and broods its young in the inner demibranch.  The gonads are ovotestes with oogenic and spermatogenic regions in each ascinus.  The sperm are biflagellate, a condition unique in the Bivalvia to triploid asexual corbiculids.  Gametogenesis was continuous and did not exhibit a seasonal pattern.  In contrast, spawning and incubation of embryos was limited to the warmer months of the year.  Embryos were present in the marsupia for up to eight months of the year from mid spring to late summer.  In most years brooding started in October and was finished by May of the following year.  C. australis develops through a highly modified veliger larva.  These larvae have a vestigially ciliated velum which is not used for swimming or particle capture.  The velum is covered by microvilli and it is suggested that the velar epithelium may be specialised for nutrient uptake in the marsupial environment.  C. australis produces several clutches each year and the young are released as advanced juveniles with a well-developed foot.  Reproductive output was strongly influenced by habitat trophic status.  The suite of life history traits exhibited by C. australis: hermaphroditism, potential for self-fertilization/androgenesis, brooding progeny to the crawl-away juvenile stage and a high reproductive output, provide for rapid recolonization and population growth in this clam which typically inhabits disturbance prone sandy lotic habitats.
 

Comparison of Morphological and Molecular Evidence on the Phylogeny of the Bivalvia

David C. Campbell

Department of Geological Sciences, CB 3315 Mitchell Hall, UNC-Chapel Hill, Chapel Hill, NC 27599-3315, U.S.A.  E-mail:  bivalve@email.unc.edu

Although DNA sequencing potentially provides enormous amounts of new data for phylogenetic analyses, DNA-based studies so far have not reached a consensus on the phylogeny of the Bivalvia, and often yield results in conflict with the consensus from morphological data.  Likewise, morphology-based studies are often in conflict with each other.  The present study analysed the entire 18S gene sequence for representatives of all living orders and a wide range of superfamilies and compared this to published morphological analyses and unpublished data, including revisions of the published analyses. The DNA-based analyses provided greater agreement with morphological data than many earlier studies, probably reflecting the increased taxonomic coverage and longer DNA sequences.  All subclasses and almost all orders were recognized as monophyletic.  Myoida, however, appears to be polyphyletic, in agreement with some morphology-based hypotheses.  Likewise, many of the relationships among the orders and subclasses suggested by the DNA have been previously proposed on the basis of morphological studies.  However, some conflicts remain to be settled by further study.
 

Phylogenetic Significance of Shell and Ligament Micro-Structure in Silurian Bivalves from Gotland, Sweden

Joseph G. Carter and David Campbell

Department of Geological Sciences, University of North Carolina, Chapel Hill, NC 27599-3315, U.S.A.  E-mail:  clams@email.unc.edu.

Recrystallized bivalves from the Upper Silurian Mulde beds of Djupvik, Gotland, contain excellent relict shell microstructure, including mineralized ligament layers. This preservation may reflect recrystallization under slightly reducing conditions in which oriented internal organic matrices are initially preserved in the diagenetic calcite.  Etching the calcite reveals their former positions.  The Mulde praenuculids, nuculids, and malletiids were characterized by a well-mineralized, submarginal simple ligament, and the nuculids also had a well-mineralized internal resilium.   Both nacreous and non-nacreous nuculids had appeared by this time. The ctenodontid Tancrediopsis foreshadows acharacid solemyoideans in its combination of a short, cylindrical, parivincular ligament, a nacreous interior, and possibly also an organic-rich outer shell layer.  Colpomya and Aleodonta had nacroprismatic shells and multiple simple ligaments, thereby confirming a close prelationship between the Modiolopsoidea and the early Mytiloidea as well as the Pterioida and Cyrtodontoida. Except for the Mytiloida, Evyana is unique among early pteriomorphians in combining a duplivincular ligament with an entirely aragonitic, nacroprismatic shell.
 

A Mechanical Model for Rib Formation in Ostreiodea

Antonio G. Checa and Antonio P. Jiménez

Departamento de Estratigrafía y Paleontología, Universidad de Granada, 18071 Granada, Spain.  E-mail:  acheca@goliat.ugr.es

Longitudinal ribs in bivalves run from the umbo to the margin; they are perpendicular to growth lines at the shell centre and become progressively more oblique towards the anterior and posterior ends. Each rib reflects the ontogenetic trajectory of the mantle sector forming it. Ostreoidea (Ostreidae and Gryphaeidae) do not match this pattern in that their ribs are perpendicular to growth lines throughout the whole shell. This implies that only in the shell centre are ribs truly longitudinal whereas towards the sides they curve lateralwards. Each rib is formed either by a laterally migrating mantle portion or by different portions of it. Oyster rib features are consistent with a mantle which extrudes perpendicular to the shell margin each time a new growth increment is to be secreted; upon extrusion the mantle margin increases its length disproportionately compared to the straight length of the shell margin, i.e., excluding the folds. This causes wrinkling along axes perpendicular to the margin, i.e., the lengthening direction. In this view, oyster ribs are purely mechanical structures whose number, size and position are not genetically fixed, but rather depend upon the mechanical properties of the mantle. This explains the high variability and irregularity of oyster ribbing patterns. This mode of rib construction contributes to phenotypic plasticity, which enables Ostreoidea to encrust a large variety of irregular substrata. The above model is supported by the homogeneous nature of the oyster mantle, unlike other ribbed bivalves in which each rib is formed by a specialised mantle protrusion.
 

A Second Look at Eastern Pacific Recent Species of the Bivalve Genus Gari

Eugene V. Coan

Department of Invertebrate Zoology, California Academy of Sciences, Golden Gate Park, San Francisco, California 94118-4599, U.S.A.   Mailing address: 891 San Jude Avenue, Palo Alto, California, 94306-2640, U.S.A; also Research Associate, Santa Barbara Museum of Natural History and Los Angeles County Museum of Natural History.
E-mail:  gene.coan@sierraclub.org

A study has been conducted of the type and other material of the Recent eastern Pacific species of the bivalve genus Gari.  There are seven species of Gari (Gobraeus).  (1) Gari (G.) californica (Conrad, 1849) (synonyms: Psammobia rubroradiata Carpenter, 1864; P. lilacina Wilkins, in Palmer, 1958 [in synonymy]) occurs from Kachemak Bay, Alaska, to Bahía Magdalena, Baja California Sur, Mexico, but with a gap between Puget Sound and Mendocino County, California.  Based on the material currently available in the United States, it cannot be distinguished from the northwestern Pacific G. kazusensis (Yokoyama, 1922), which is also regarded as a synonym, along with G. k. atsumiensis Hayasaka, 1961.  (2) Gari (G.) fucata (Hinds, 1845) (synonym: Siliquaria edentula Gabb, 1869), occurs from Ventura County, California, to Punta Eugenia, Baja California Sur, Mexico, and perhaps as far south as Bahía Magdalena.  (3) Gari (G.) lata (Deshayes, 1855) (synonym: Psammobia regularis Carpenter, 1864), occurs from Bahía Magdalena, Baja California Sur, Mexico, throughout the Gulf of California, south to Santa Elena, Ecuador.  Records of Gari regularis from northern Baja California are based on misidentified small, elongate, inflated specimens of G. californica.  (4) Gari (G.) maxima (Deshayes, 1855) occurs from Mazatlán, Mexico, to Panama.  (5) Gari (G.) panamensis Olsson, 1961, occurs from the central Gulf of California to Playas, Ecuador.  (6) Gari (G.) solida (Gray, 1838) (synonyms:  Psammobia solida Philippi, 1844; P. crassa Hupé, 1854), occurs from Arica to Rio Inio, Chile.  (7) A probable new species of Gari (G.) occurs in the Galapagos Islands, thus far represented by only a single, small, broken specimen.  An eighth species, Gari (Dysmea) helenae Olsson, 1961, occurs from Laguna Ojo de Liebre, Baja California Sur, Mexico, throughout the Gulf of California, south to Isla Salango, and the Galapagos Islands, Ecuador.  Its relationship to the western Atlantic Gari circe (Mörch, 1876) and G. linhares Simone, 1998, remain to be resolved.  Several lectotype designations will be made, and a list will be provided of New World Recent and fossil taxa that have been placed in Gari.
 

 Phylogeny: The Key to Bivalve Taxonomy

John C.W. Cope

Department of Earth Sciences, Cardiff University, PO Box 914, Cardiff CF1 3YE, U.K.
E-mail:  copejcw@cardiff.ac.uk

In the 30 years since publication of the bivalve Treatise, important new faunas have been described, from the early and mid Cambrian and from the early and mid Ordovician. These contain significant new forms, including some long-ranging intermediate groups, that indicate the relationships between the principal bivalve clades. We now know that the earliest bivalves were palaeotaxodonts, resolving the controversy over the primitive bivalve dentition, and that the major phase of bivalve diversification followed on from the evolution of the Subclass Autobranchia, in the latest Cambrian or earliest Ordovician  (a time interval that coincides with a major hiatus in the bivalve record). The principal division of the Class is into two subclasses, Protobranchia and Autobranchia; links between the two can be demonstrated in the early Ordovician.  Major divisions of each subclass are recognised as superorders. Within the Protobranchia, the Palaeotaxodonta developed specialist food-gathering palps and an enlarged foot.  They diversified to produce distinct forms living symbiotically with sulphur-oxidizing chemoautotrophic bacteria;  this allowed colonization of soft substrates and produced two stocks: the deeply infaunal anteriorly elongate solemyoids (Lipodonta) and the semi-infaunal and epifaunal Cryptodonta. The Autobranchia, initially identified by strongly asymmetrical hinges, diverged in three directions, each characterized by distinctive hinges. The Anomalodesmata developed a strong ligamental insertion and largely lost their dentition. The Trigonioids were characterized by denticulate teeth and rapidly regained greater symmetry, whilst the Heteroconchia, with a crossed-lamellar shell, bifurcated early into the glyptarcoids leading to the neotaxodonts and pteriomorphians, and the actinodontoids leading to the mainstream heteroconchs.
 

Evolution of Taxonomic Diversity Patterns in Marine Bivalves

J. Alistair Crame

British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, U.K.
E-mail: JACR@pcmail.nerc-bas.ac.uk

Bivalves have been fundamental to the development of our understanding of large-scale biodiversity patterns in the mariine realm.  In particular, they have helped delineate steep latitudinal gradients in each hemisphere and high-diversity foci in the central American and Indonesian-Philippines regions, respectively.  A new global compilation of some 29 regional bivalve faunas again picks out these patterns but suggests that there is in fact a considerable degree of north-south asymmetry.  Whereas northern gradients tend to be steeper and more regular, southern ones are influenced by a more complex pattern of inter-regional variation.  It is likely that bivalve latitudinal and longitudinal gradients have been formed by a combination of equilibrium and non-equilibrium processes.  Amongst the latter, it is now clear that the steepest gradients occur in the youngest bivalve taxa.  This relationship is particularly strong within the heteroconchs, the youngest, and largest, of seven major extant clades.  It can be concluded that the Late Cretaceous - Cenozoic infaunalization of the Bivalvia was essentially a low-latitude phenomenon, and that many heteroconch groups have yet to become fully established within high-latitude and polar ecosystems.  Observations on a series of fossil latitudinal gradients indicate that tropical high bivalve diversity has been consistently underpinned by infaunal taxa.  A comparison of Late Palaeozoic, Mesozoic and Recent gradients gives some indication of the rate at which clades may have been displaced into high latitudes.
 

Evolution of Shape throughout the Lifespan of an Infaunal Bivalve Genus:  Cenozoic Spissatella (Crassatellidae) from New Zealand

James S. Crampton(1) and Phillip A. Maxwell(2)

(1) Institute of Geological and Nuclear Sciences, P.O. Box 30-368, Lower Hutt, New Zealand.  E-mail:   j.crampton@gns.cri.nz.
(2)  Bathgates Road, R.D. 10, Waimate, New Zealand

Spissatella is a moderately speciose genus of non-siphonate, shallow water Crassatellidae from the Late Eocene to Late Miocene of New Zealand and Australia.  This study uses Fourier shape analysis to examine ontogenetic, intra-"populational", and evolutionary changes in outline shape in 300 individuals from 20 collections spanning the Eocene to Miocene.  Outline shape was probably a key target of evolutionary selection, given its relationship to speed and depth of infaunal burrowing and, therefore, survival in the face of predation and environmental perturbation.

Results demonstrate that, over a 20 Ma period, the greatest component of shape variation was related to ontogenetic development and that evolution in Spissatella was largely the result of heterochronic processes operating at the post-larval stage.  Furthermore, size and shape covary and it appears that these two traits were not selected for independently: to vary shape, it was necessary to vary size, or vice versa.  There was little evolution away from the basic ontogenetic plan and, where detected, such evolution may have been a consequenceof heterotopy (changes in the spatial patterning of growth fields).  The basic ontogenetic plan is diagnostic at the generic level.  The data also demonstrate that morphological variance is inversely correlated with water depth; that there is little evidence of morphological stasis at the sampling resolution; and that there are no long-term evolutionary trends in size or shape.  Together, these results suggest that throughout the lifespan of this clade, evolution was dominated by gradual change in response to shifting environmental ranges and within strict developmental constraints.
 

Genetic Characterisation of Mytilus galloprovincialis Populations with Nuclear DNA Markers

Claire Daguin(1), François Bonhomme(1) and Philippe Borsa1(2)

(1) Laboratoire Génome, Populations, Interactions, UPR 9060 CNRS, Université de Montpellier II, Centre National de la Recherche Scientifique, 34200 Sete, France.
E-mail : daguin@crit.univ-montp2.fr
(2) Institut de Recherche pour le Développement, Montpellier Station Méditerranéenne de l’Environnement Littoral, 1 Quai de la Daurade, F-34 200 Sete, France

The genetic relationships among Mytilus galloprovincialis populations over their world-wide range were investigated using polymerase chain reaction (PCR)-amplified nuclear DNA markers. We used long-range polyacrylamide gel electrophoresis for characterising a high level of intron-length polymorphism at the actin gene locus mac-1, the most polymorphic DNA marker known to date in Mytilus. Significant differences in allelic frequencies were observed between north-eastern Atlantic and Mediterranean M. galloprovincialis populations, but no variation was detected within either the Atlantic or the Mediterranean/Black Sea. M. edulis alleles were present at low frequencies in Atlantic M. galloprovincialis populations, and to a lesser extent in Mediterranean populations. Previous allozyme and morphological surveys have shown that M. galloprovincialis is also present in California, Northeast Asia, South Africa and Australia/New Zealand. The genotypic characterisation of non-European populations at locus mac-1 revealed that the origin of Korean M. galloprovincialis is the Mediterranean whereas the origin of South African M. galloprovincialis is the Atlantic. Californian M. galloprovincialis were found to be close to, although slightly different from, Mediterranean M. galloprovincialis. We also report for the first time the occurrence of M. galloprovincialis, of Mediterranean origin, in central Chile. All the foregoing M. galloprovincialis populations were also characterised at the polyphenolic adhesive protein gene locus Glu 5’, which is supposed to be diagnostic between M. edulis, M. galloprovincialis and M. trossulus. Glu 5’ data were generally in accordance with mac-1 data.

 A Fresh Look at Jurassic Retroceramidae and their Mode of Life

Susana E. Damborenea (1) and Paul A. Johnston (2)

(1) Departamento Cientìfico Paleontologìa Invertebrados, Museo de Ciencias Naturales, La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina. E-mail: susanad@mmance.cyt.edu.ar
(2) Royal Tyrrell Museum of Paleontology, Box 7500, Drumheller, Alberta, Canada T0J 0Y0. E-mail: pjohnston@mcd.gov.ad.ca

The taxonomic relationships and life habits of retroceramids are reappraised on evidence from Middle Jurassic Retroceramus species from central western Argentina and elsewhere.  The family Retroceramidae is removed from the Pteriomorphia and relocated within the Superfamily Inoceramoidea in the Subclass Cryptodonta because: a) details of the retroceramid ligamental area reveal a linear growing margin, as in inoceramids, and not sinusoidal, as in Isognomon; b) posterior pedal muscle scars are comparatively large, subcentrally placed, and well separated from the relatively small, distally placed posterior adductor, fitting the inoceramid and praecardioid patterns, not that of pteriomorphs; c) several species show geniculations of shell profile resulting from changes in shell convexity and ornament during ontogeny; and d) main ornament consists of pronounced comarginal rugae affecting both inner and outer shell surfaces. Shell shape and ornamentation indicate that Retroceramus species were orthothetic, probably semi-infaunal or epifaunal, and lived on mud-grade substrates in poorly oxygenated settings, as supported by taphonomic evidence. Occurrence of these bivalves with ammonoids only, or with few other benthonic megafauna, in deposits originated in dysaerobic environments suggests that retroceramids may have harboured as symbionts chemosynthetic sulphophilic bacteria, as already proposed for inoceramids on various grounds (isotopic, sedimentological, etc.) by other authors. Furthermore, peculiar modifications of the ventral region of some Retroceramus species studied, suggest a thin, flexible, ventral flap-like extension of the shell margin which might have been related to the presence of a ventral, sulphide-pumping organ.
 

 Palaeogeographic Distribution Patterns in Upper Cretaceous Bivalves

Annie V. Dhondt

Department of Palaeontology, Royal Belgian Institute of Natural Sciences, Vautierstraat 29, B - 1000 Brussels, Belgium.  E-mail:  dhondt@d5100.kbinirsnb.be

The Upper Cretaceous is a period of extensive transgressions.  In the Cenomanian NW Europe was largely covered by shallow seas. The faunas from the Upper Greensand facies from S. England also occur in W. France, Belgium, Germany (Westphalia and Saxony), Czechia, Poland, Western and Eastern Russian Platform, Moldavia and into Central Asia.
Coeval transgressive pulses distributed mainly oysters on the northern margin of the Tethys, from the Paris Basin to Central Asia.  Part of these strata contains rudists. Especially in the fore and back reefs of these rudist bioherms specific faunal associations are present (N. Italy and the Balkan).  In Northern Africa (and Sicily) the Cenomanian is characterised by extensive oyster facies (also containing plicatulids, pectinids (Neithea and large Chlamys), limids). Many taxa extended from NW South America, from Mexico-Texas across N. Africa and W. Asia, into Central Asia.  The Cenomanian/Turonian regression with its anoxic facies in less shallow deposits) resulted in the extinction of many bivalve taxa. Only progressively new taxa replace them from the Turonian onwards.  In N. Europe White chalks appeared in the Turonian: a not very shallow deposit showing specific evolution between the Turonian and the Campanian (Lower Maastrichtian) (in pectinids (Chlamys, Microchlamys), limids (Plagiostoma, Limatula, Limea), spondylids and inoceramids (especially Mytiloides). In the Turonian - Campanian, in more littoral environments, oysters, pectinids (Neithea), limids (Ctenoides) and in the Tethys the evolution was different (example: N. Tethys deposits in the Gosau). The S. Tethys continued to have an oyster facies in shallow environments.  The S. American faunas contained elements common with North Africa until the Campanian, but endemic elements already occur in the Santonian (?) Campanian Maastrichtian.  In N. America a cosmopolitan fauna is known in the Albian-Cenomanian, but later the Western Interior, Texas and the Atlantic Coast contain more endemic elements. The W. Coast faunas from the Turonian onwards are Pacific, and closer to the Japanese and Eastern Siberian faunas.

 New Perspectives on the Gills and Pallial Organs of Freshwater Mussels (Paleoheterodonta: Unionoida: Unionoidae)

Ronald V. Dimock, Jr (1), Richard A. Tankersley (2) and Maria Byrne (3)

(1) Department of Biology, Wake Forest University, Winston-Salem, NC 27109, U.S.A.
E-mail:  dimock@wfu.edu
(2) Department of Biological Sciences, Florida Institute of Technology, Melbourne, FL 32901, U.S.A.
(3) Department of Anatomy and Histology, University of Sydney, NSW 2006, Australia

Video endoscopy provides real-time in vivo visualization of the structure and function of pallial organs of bivalves that here-to-fore could not adequately be imaged. For example, insertion of an endoscope into the supra-branchial chamber enables viewing of the interior of gills, revealing 3-dimensional detail that previously could only be interpreted from histology or dissection.  We have used this technique to examine the gills and associated structures of the Hyriidae, Margaritiferidae and Unionidae, families that exhibit the three larval brooding conditions of the Unionoidea. Endobranchous species utilize the inner demibranchs for the retention of developing glochidia, whereas tetragenous species employ all four, and ectobranchous mussels use the outer demibranchs.  Differential use of gills as marsupia is accompanied by variation in the interlamellar tissue connections characteristic of the eulamellibranch ctenidium.  Endobranchous hyriids have perforated interlamellar septa in the marsupial demibranch.  The tetragenous margaritiferids have either simple interlamellar tissue junctions or obliquely oriented septa.  Anodontine unionids have numerous vertical non-perforated septa in the marsupial demibranch.  The separation of infra- from supra-branchial chambers is either partial via diaphragmatic septa (margaritiferids) or a perforate gill diaphragm (hyriids) or complete by the fusion of gills to the posterior mantle (unionids).  In vivo imaging revealed the dynamic association of labial palps with the demibranchs.  In addition, hyriids, unlike margaritiferids and unionids, are shown to have a prominent renal papilla adjacent to the genital opening in the supra-branchial space above the innner demibranch. These imaging techniques provide new insights for functional and phylogenetic considerations.

Videotapes from this study may be viewed during the Feeding Workshop.
 

 Burying Depth of Macoma balthica represents a Flexible Anti-Predation Behaviour

Pim Edelaar (1,2) and Diliana Welink (2)

(1)  Netherlands Institute for Sea Research, Texel, The Netherlands.
 E-mail:  edelaar@nioz.nl.
(2) Center for Ecological and Evolutionary Studies, University of Groningen, The Netherlands

Macoma balthica is a small tellinid that buries in soft sediment. Burying depth shows extreme variability, and the adaptive value of the variation was studied. Burying depth is usually explained in terms of a trade-off between food and safety, as deeply buried individuals supposedly are at smaller risk of predation, but suffer from a decrease in food intake. Our experiments show that individuals fixed at shallow depths grew more then those fixed at greater depths, and when food is provided, the individuals bury less deep. Individuals exposed to predator cues bury deeper, indicating a benefit of burying deeper when in danger of predation. Burying depth can be largely explained by the size of the individual. Up to a shell length of about 15 mm individuals bury progressively deeper, but from then onwards burying depth diminishes rapidly. Are these larger individuals in some way constrained? Both small and large individuals were exposed to either caged small or large Shore crabs (Carcinus maenas), or control empty cages. The large individuals did bury deeper when crabs were present, showing that such a constraint is not present. Interestingly, they increased burying depth mostly when the predator was a large crab. Small individuals already showed a large increase in burying depth when exposed to small crabs. Small crabs can open small individuals but cannot eat large individuals, whereas large crabs can. These experimental data confirm that burying depth is a trade-off between food intake and risk of predation. Such a flexibility in anti-predation behaviour will not only affect survival rates, but also growth and reproductive rates.
 

Correlation of Protein Synthesis with Morphological Changes during Metamorphosis of the Glochidia of Utterbackia imbecillis (Unionoida: Unionidae)
Ginger R. Fisher and Ronald V. Dimock, Jr.

Department of Biology, Wake Forest University, Winston-Salem NC 27109, U.S.A.
E-mail:  mackgr5@wfu.edu

The period of metamorphosis from glochidia to juvenile is a critical time in the life history of unionid mussels; however very little is known about the molecular and morphological changes that accompany this transition. Glochidia were isolated from gravid parental mussels and cultured in vitro through metamorphosis. The rate of RNA, DNA and protein synthesis was measured daily over the 8-day development period. There was a significant decrease in the rates of synthesis during metamorphosis as compared to pre-cultured glochidia and juvenile mussels. Once the animals entered the metamorphic period, the level of cell division and protein synthesis increased steadily for three days and then decreased dramatically between days three and four. The fourth day of metamorphosis was characterized by low rates of RNA, DNA and protein synthesis. From day four until the end of metamorphosis, the synthesis levels steadily increased. The developing animals were examined histologically to determine what morphological changes correlated to the molecular changes we observed. During metamorphosis the glochidial tissues are degraded and the animals develop the juvenile morphology. Following metamorphosis the juveniles possess a foot, two adductor muscles, a stomach, gastric shield, crystalline style, and gill bars. This study provides a detailed description of the timing and development of these features and is the first attempt to use both molecular and morphological characters to describe the process of metamorphosis in unionid mussels.
 

The Systematics of Planktomya, a Bivalve Genus with Teleplanic Larval Dispersal

Serge Gofas

Departamento de Biologia animal, Facultad de Ciencias, Universidad de Málaga, E-29071 Málaga, Spain.  E-mail: sgofas@uma.es

The small bivalve Planktomya henseni Simroth, 1896 (type species of Planktomya Simroth, 1896), originally described as a pelagic species from a plankton tow in the North Atlantic and later recognized as a benthic Caribbean species, is shown to be also present in the Eastern Atlantic, in the islands off West Africa. The morphology of adults and protoconchs is redescribed and figured taking into account new material.  The Eastern Atlantic species Nesis prima Locard, 1899 (type species of Nesis Locard, (1899), is assigned to Planktomya on the basis of larval shell morphology. The generic name Planktomya has precedence over Nesis (preoccupied), so that the replacement name Monterosatus Beu, 1971 is not necessary.   Planktomya prima was described from the continental shelf of Bay of Biscay,  and is shown to occur south to Senegal and Guinea. A further species of Planktomya is described from Southern Angola.  The systematic position of Planktomya is discussed. It is concluded, on the basis of shared character states of the hinge, ligament and pallial line, that it should be placed in the Montacutidae, in the vicinity of Tellimya Brown, 1827. The current placement of Monterosatus in the Mesodesmatidae is rebutted. The strategy of larval dispersal, with teleplanic larvae, is briefly discussed and noted to be an extreme case of r-strategy, where large quantities of larvae never reach the shelf to metamorphose.
 

Testing Models of the Relationships of the Extant Anomalodesmatans

Elizabeth Harper(1), Elizabeth Hide(2) and  Brian Morton(3)

(1) Department of Earth Sciences, Downing Street, Cambridge, CB2 3EQ, U.K.
E-mail:  emh21@cus.cam.ac.uk
(2) National Museums of Scotland, Chambers Street, Edinburgh EH8 9EJ, U.K.
(3) The Swire Institute of Marine Science and Department of Ecology and Biodiversity, The University of Hong Kong, Hong Kong

The Anomalodesmata are a fascinating bivalve sub-class, with a long evolutionary history dating back 500 million years.  Nearly 15% of all the bivalve families that have ever lived are classified within the Anomalodesmata and yet the relationships between its constituent taxa (both living and fossil families) remain obscure.  The 13 families of extant anomalodesmatans form an extremely diverse group whose members account for some of the rarest and most specialised of Recent bivalves. They exploit a wide range of habitats from shallow to deep sea, as shallow and deep burrowers in soft sediments, or attached to hard surfaces either by byssal threads or by permanent cementation.  They include the remarkable "septibranchs" which are voracious predators in the deep-sea and also the enigmatic tube-dwelling clavagellids.  This plethora of life habits has led to an equal variety of overall morphologies which has confounded analyses of their phylogenetic inter-relationships based on single character systems and these problems appear to have been further exacerbated by convergent and parallel evolution. In order to overcome these obstacles we have used an approach which takes into account the results of a cladistic study (based on both hard-part and tissue characters) and an analysis of the fossil record.

Phylogeny and Taxonomy of Cementing Triassic Bivalve Families (Prospondylidae, Dimyidae and Ostreidae)
Michael Hautmann

Institute for Palaeontology, University of Würzburg, Pleicherwall 1, 97070 Würzburg, Germany.  E-mail: hautmann@mail.uni-wuerzburg.de

Cementing bivalves belonging to the families Prospondylidae, Plicatulidae, Dimyidae and Ostreidae are an important constituent of Upper Triassic shallow marine ecosystems. Based on new material from the Upper Triassic Nayband Formation of east-central Iran and on type-material from the Alpine Triassic, the taxonomy and phylogeny of these families is examined. The Plicatulidae developed from an ancestor within the Prospondylidae by forming strong crurae, which allowed a reduction of the lateral part of the ligament. Their hinge was later modified by shifting resilifer and crurae in a ventral direction and by forming a secondary ligament dorsally. Only slight modifications of the shell led to the Spondylidae, which are the (post-Triassic) adelphotaxon of the Plicatulidae. For the monophylum consisting of these three families, the name Spondyloidea Gray, 1826 is available. Contrary to some recently proposed classifications, a direct relationship to the morphologically similar Dimyidae and Ostreidae is unlikely.
 

Comparative Sperm Ultrastructure in Pteriomorphian Bivalves with Special Reference to Phylogenetic and Taxonomic Implications

John M. Healy and Jennifer L. Keys

Department of Zoology and Entomology, University of Queensland, Brisbane,
Australia, 4072.   E-mail:  jhealy@zoology.uq.edu.au   jkeys@zoology.uq.edu.au

Comparative sperm ultrastructure reveals that the Pteriomorphia exhibit the widest diversity of acrosomal morphology to be seen in any bivalve subclass. Pteriomorphian spermatozoa, like those of most other bivalves, are of the classic aquasperm type (conical acrosomal vesicle, short to rod-shaped nucleus, short midpiece composed of two centrioles and a ring of spherical mitochondria, simple flagellum). Whereas most other bivalve subclasses show at least some defining acrosomal feature(s), this does not appear to be the case within the Pteriomorphia. This raises the question as to whether the Pteriomorphia are truly monophyletic or simply more experimental in relation to their sperm morphology. Pteriomorphian superfamilies not only differ substantially from each other in sperm morphology but also show varying levels of diversity between and within families and genera. In the Ostreoidea the spermatozoa are remarkably uniform in their structure, with the exception of the apical region of the acrosomal vesicle which shows potentially useful generic-level variation in the Ostreidae. A very close relationship between the Pectinidae and Spondylidae of the Pectinoidea is demonstrated, with more distant connections to the Ostreoidea, Anomioidea and Limoidea. Within the Mytilidae (Mytiloidea) there is substantial variation between supraspecific taxa especially at the subfamial level.
 

 Dissecting the Latitudinal Diversity Gradient in Marine Bivalves

David Jablonski(1), Kaustuv Roy(2) and James W. Valentine(3)

(1) Dept. Geophysical Sciences, University of Chicago, 5734 S. Ellis Avenue, Chicago,
IL 60637, U.S.A.  E-mail:  djablons@midway.uchicago.edu
(2) Dept. Biology, University of California, San Diego, CA 92093-0116, U.S.A.
(3) Dept. Integrative Biology, University of California, Berkeley, CA 94720, U.S.A.

The latitudinal diversity gradient, with maximum taxonomic richness in the tropics, is one of the most pervasive biological patterns, but its basic configuration and its temporal dynamics remain poorly known for marine organisms. An analysis of 945 bivalve species from the Eastern Pacific continental shelf (northwest Peru to the north coast of Alaska in the Arctic Ocean) confirms the existence of a latitudinal gradient in taxonomic diversity. This gradient is strong in both infaunal and epifaunal bivalves (albeit with different slopes), contrary to Thorson’s (1952, 1957) long-standing hypothesis that only epifaunal groups increase in diversity towards the tropics. Protobranch bivalves do not show a latitudinal trend, and this may reflect the feeding habits of the adults or of the larvae. Given the latitudinal patterns in species richness, and the near-complete turnover of species from poles to equator, it is striking that the size-frequency distribution of species at the provincial level does not change with latitude. As with gastropods in both the Eastern Pacific and western Atlantic, the overall bivalve diversity gradient is significantly correlated with sea surface temperature, even when the effects of latitude are factored out. This supports the hypothesis that biological diversity gradients depend heavily on the total or average energy input, which would be a complex function of solar input (both mean and variance) and productivity. Preliminary data suggest that the species (and genus) extinction and origination rates are higher in the tropics than in the Arctic, but the modal body sizes of bivalve faunas have not been evolutionary attractors.
 

 Palaeoenvironmental Reconstruction from Ontogenetic Records in the Shell of the Queen Scallop, Aequipecten opercularis (L.)

Andrew L.A. Johnson(1), J.A. Hickson(1), J. Swan(1), M. Brown (1), T.H.E. Heaton(2), P.S. Balson(2) and S. Chenery (2)

(1)  Department of Earth Sciences, University of Derby, Derby DE22 1GB, U.K.  E-mail:  A.L.A.Johnson@derby.ac.uk
(2)  British Geological Survey, Keyworth, Nottinghamshire, U.K.

Aequipecten opercularis is a widespread scallop, occurring at present from northern Norway to the Adriatic, and extends back to the Miocene. Studies on animals cultured under monitored, semi-natural conditions show that the the oxygen of shell carbonate is incorporated in isotopic equilibrium with ambient seawater (hence preserving a record of temperature variation) and that at least in the first year growth is rapid and only interrupted for brief intervals during winter (hence providing for reconstruction of almost the full seasonal temperature range). Seasonal variation is evident in shell magnesium concentration and microgrowth-increment width so there is scope for independent verification of at least the temporal basis of isotopically-determined temperature changes. ‘Summer’ ?O18 values from sub-fossil shells of the North Sea Basin are closely comparable to those from modern shells but ‘winter’ values are somewhat enriched, suggesting either cooler temperatures or (more probably) enhanced food supply/ability to feed, resulting in more continuous winter growth and registration of the very lowest temperatures experienced. Microgrowth-increment data support palaeobiogeographic evidence that Pliocene marine temperatures were substantially higher than at present, but isotopic evidence is contradictory. At the very least this argues for multiproxy investigations of palaeotemperature, and may indicate that factors other than temperature (e.g. food supply) are important in determining the latitudinal ranges of taxa.

Contrasting Structure and Morphogenesis of Ligaments in Cryptodonta and Early Pteriomorphia (Mollusca; Bivalvia)

Paul A. Johnston (1) and Christopher J. Collom (2)

(1) Royal Tyrrell Museum of Palaeontology, P.O. Box 7500, Drumheller, Alberta, T0J 0Y0, Canada.  E-mail: pjohnston@mcd.gov.ab.ca
(2) Department of Earth Sciences, Mt. Royal College, 4825 Richard Road, Calgary, Alberta, T3E 6K6, Canada

The bivalve subclass Cryptodonta is characterized primitively by an opisthodetic monovincular ligament area with horizontal growth lines and a ventrally accreting, linear, growing margin, but without differentiated zones for insertion of lamellar and fibrous ligament components. Improbably homogeneous, monovincular ligaments may instead have been constructed as in Nucula, with a medial wall of lamellar ligament separated from the hinge plate on either side by a layer of fibrous ligament (granular in Nucula).  But unlike Nucula, dorsal placement of the ligament in cryptodonts indicates a predominantly tensile function. Some primitive pteriomorphs such as Cyrtodonta show horizontally striated ligament areas that mimic monovincular ligaments; however, the striations are grooves and ridges, not growth lines, as those ventralmost do not extend the length of the ligament area.  These ligaments are duplivincular and differ fundamentally from monovincular ligaments both morphogenetically and functionally.  Orientation of grooves on duplivincular ligament areas, whether inclined or subhorizontal, is a simply a vector determined by the rate of propagation of secretory waves along the mantle isthmus and the rate of accretion of the ventral margin of the ligament area. We can now document well-preserved monovincular ligament areas in cryptodonts representing every geologic period from Upper Ordovician “Vlasta” americana to Upper Cretaceous Tenuipteria.  Such continuity of monovincular ligament construction through time is important for our phylogenetic arguments that link exclusively Mesozoic cryptodont groups such as Buchiidae, Pergamidiidae, and Halobiidae with older Paleozoic praecardioid cryptodonts, rather than with the Pteriomorphia. An apparent monovincular ligament in the earliest known bivalve Pojetaia (Tommotian) raises the interesting possibility that these ligaments are primitive for the Bivalvia.
 

Ontogenetic Age Determination and Evolutionary Patterns in Gryphaea from the British Jurassic

Douglas S. Jones

Florida Museum of Natural History, University of Florida, Gainesville, FL 32611,
U.S.A.  E-mail:  dsjones@flmnh.ufl.edu

Few bivalves have played a more significant role in evolutionary studies than the coiled Jurassic oyster, Gryphaea.  Since the seminal work of Trueman (1922), we have known that the Lower Jurassic Gryphaea lineage of Britain is characterized by phyletic size increase and heterochronic change in shape. Subsequent work by Hallam indicated that this increasing size was accompanied by an overall juvenilization of form.  The evolution of shape represents a clear case of paedomorphosis. However, without the ability to standardize samples by common age or stage of development, it remained impossible to specify the mode of heterochrony responsible for this paedomorphic result. Johnson’s (1993, 1994) reanalyses of evolutionary patterns in this lineage identified the same procedural problem discussed earlier by Gould (1972) and found to be at the heart of the coiling debate - improper standardization when comparing ancestors and descendants.  Fortunately, annual growth increments revealed in shell cross-sections permit ontogenetic age and growth rate determinations for sample populations throughout the lineage.  Growth curves indicate that phyletic size increase in Liassic Gryphaea is achieved by faster growth and not by a hypermorphic extension of time to maturity.  The well-known decrease of coiling in the upper part of the sequence, accompanied by increasing size and juvenilization of form, represents a true case of neoteny (Jones and Gould, 1999).  An independent series of Gryphaea from the Middle-Upper Jurassic reveals a strikingly different pattern.  Direct measurement of ontogenetic age using periodic growth increments provides a powerful mechanism to assess heterochronic style in evolving bivalve lineages.
 

Giant Bivalves from a Barremian (Early Cretaceous) Seep System in Wollaston Forland, Northeast Greenland

Simon R.A. Kelly, Eric Blanc, Simon P. Price and Andrew G. Whitham

Cambridge Arctic Shelf Programme, Gravel Hill, Huntingdon Road, Cambridge CB3 ODJ, U.K.

Anomalous mound-forming limestones, here termed the Kuhnpasset Beds, occur within Barremian mudstones from Wollaston Forland.  They contain a locally abundant and unusual faunal assemblage, dominated by bivalves. The taxa include a giant permophorid gen. et sp. nov., reaching 300 mm length, lucinaceans including Cryptolucina, Solemya sp. and drift-wood with the wood-boring Turnus sp.  The form of the mounds with calcite cemented tube systems, associated calcite crusts and laminated void fills probably indicates a cold-seep complex. Although shell preservation is siliceous, which precludes geochemical studies concerning their origin, it is probable that the seeps are methane-related.  It is believed that the mounds formed on a mid- to outer shelf situation during the period of quiescence following earlier Cretaceous extensional rifting on the eastern Greenland passive Atlantic margin.  The underlying faults may have contributed to hydrocarbon reservoir formation.  Seepage along faults through the seals of the reservoirs was active during Barremian but had ceased by Aptian time.
 

Relevance of Sperm Ultrastructure to the Classification of Giant Clams (Mollusca, Cardiodea, Cardiidae, Tridacninae)

Jennifer L. Keys and John M. Healy

Department of Zoology and Entomology, The University of Queensland, Brisbane, Australia, 4072. E-mail: jkeys@zoology.uq.edu.au;   jhealy@zoology.uq.edu.au

Sperm ultrastructure of six out of eight of the living species of giant clams (traditionally regarded as a distinct family Tridacnidae, superfamily Tridacnoidea) is examined and the data discussed firstly in relation to other bivalve sperm and secondly in relation to recent taxonomic and phylogenetic studies on the Cardioidea. The results support the work of Schneider (1992, 1995, 1998a,b) that the Tridacnidae should be regarded as a subfamily of the Cardiidae (as Tridacninae), and are not worthy of being placed in a separate superfamily.

Tridacnine spermatozoa are all of the aquasperm type, featuring, in anterior-posterior sequence: a conical acrosomal vesicle, an oblong to rod-shaped nucleus, a short midpiece region (with a proximal and distal centriole surrounded by a cluster of four, round mitochondria) and a flagellum (axoneme of 9+2 microtubular pattern). Although the midpiece of most species follows essentially the same pattern throughout the group (a pattern seen throughout the Bivalvia), there are substantial differences between species in the shape, length and volume of the nucleus, and in the spatial relationship between the acrosomal complex and the nuclear apex. Results of our study clearly show a dichotomy within the Tridacninae between Tridacna (subgenera Tridacna sensu stricto, Persikima, Chametrachea) on the one hand and Hippopus on the other. This is based on the occurrence in the Tridacna of a prominent nuclear peg which fits into the invaginated base of the acrosomal vesicle (peg absent in Hippopus) and the presence in Hippopus of a well developed centriolar rootlet, lying lateral to but contacting the centrioles (rootlet vestigial or absent in Tridacna).  Given the occurrence of a apical protrusion of the nucleus in several investigated Cardiinae, either as a discrete peg as in Cerastoderma, or a broad bump as in Lunulicardia, the complete absence of a protrusion in Hippopus is presumably due either to secondary loss or perhaps even diphyly of the Tridacninae.  Within Tridacna, the species T. (Chametrachea) maxima and T. (C.) crocea are distinguished from other species of the genus by a strongly attenuate nucleus and a considerably smaller acrosome.  In contrast, and against expectation, T. (C.) squamosa shows acrosomal and nuclear dimensions very close to that obtained for T. (Tridacna) gigas.
 

Ecological Fidelity of Molluscan Death Assemblages

Susan M. Kidwell

Department of Geophysical Sciences, University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637, U.S.A.  E-mail: skidwell@midway.uchicago.edu

Although individual studies have yielded mixed results, a comparative analysis of marine molluscan faunas and their associated dead from 17 study areas) indicates that sedimentary death assemblages are very robust reflections of local community composition. Virtually all live species (mean 89% ± 5) are present in the local death assemblage, dead individuals overwhelmingly belong to species found living in the same habitat (mean 82% ± 10), and the rank abundances of dead species do not diverge significantly from those of live species (80% of datasets tested; p < 0.05).  Even small samples of the death assemblage thus capture basic dominance information and habitat preferences of the live fauna, with only slight differences in fidelity among environments (marshes and tidal creeks; intertidal flats; coastal embayments; open marine seafloors).  This correspondence is especially striking given the number of post-mortem processes that might act to bias such a record.  Because the species richness of a death assemblage is typically 2-3x greater than that of any single census of the local live community, inverse metrics such as “% dead species also present alive” suggest low live-dead agreement.  However, the majority of dead-only species are rare and most of the discrepancy (excess dead species richness) is evidently due to under-sampling of the live fauna.  When limits imposed by sampling are considered, true post-mortem bias from the addition of exotic and relict shells is probably less than 25% of total dead species richness, and would have little effect on abundance-based diversity measures.  In general, because of their greater numerical abundance, bivalve species are less affected than gastropod species.  Molluscan death assemblages thus provide a reliable—plus relatively rapid and inexpensive—means of assessing community composition, both for the purpose of establishing ecological baselines as well as for paleoecological analysis of ancient rocks.  Continuing work focuses on the types of species (body sizes, shell mineralogies or microstructures, life habits, habitat types) that are correlated with under- or over-representation in the sedimentary record, and in acquiring datasets from low latitudes and from areas with long-term replicate sampling of live faunas.

Pectinid Bivalve Pedum and the Amount of Surface Occupied in Host Corals (Red Sea)

Karl Kleemann

Institute of Palaeontology, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria
E-mail:Karl.Kleemann@univie.ac.at

The pectinid boring bivalve, Pedum spondyloideum (Gmelin 1791), associated to scleractian hosts, lives embedded in the coral skeleton, usually completely surrounded by live tissue (Kleemann 1990). In the northern Red Sea, off the ports of Hurgada and Safaga, and at Zabargad Island, close to the Sudan, associations of Pedum with coral hosts were observed in the field. Documentation took place by in situ colour slides. Two frame sizes,
9 x 6 cm and 19 x 13 cm respectively, were used on a Nikonos II camera with electronic flash. A small collection was used for measurements of the shells and their dwellings. The size-relation between shell and dwelling is very good. The relation between dwelling length and dwelling volume was used to determine a formula to estimate the occupied volume in the hosts from the photographs. The selection of photographs was taken under the aim (1) to document the range of host corals in generic and specific level as far as possible, and (2) to record high densities of the bivalve in certain hosts. The associations with corals Hydnophora microconos, Pavona cactus and P. varians are here recorded for the first time.
Live coral surface versus occupied coral surface (OCS) was measured in the scanned pictures, format DIN A4, by Autocad program. For easy comparison between coral hosts, values were converted to 100 cm2 coral surface. They are not convertible to larger scale, as maximum densities usually apply only to parts of the host colony, and "mean" values are derived from the available sub-samples. On various coral carpets, Pedum density ranged from 0 to 17.8 individuals m-2 (Zuschin & Piller 1997). In the 9x6 cm frames, 1.9 to 18.6 Pedum occurred per 100 cm-2. The maximum was found in a Montipora, with 12.45 % OCS. Mean density in Montipora (n = 11) was 7 100 cm-2, and the mean OCS amounted 3.8 %. In the 19x13 cm frames, density ranged from 0.4 to 10.7 Pedum 100 cm-2. The maximum occurred again in Montipora, followed by Porites (6.7 100 cm-2) and Cyphastrea (5 100 cm-2). The OCS ranged from 0.18 to 7.04 %. The latter was found in Goniastrea, and amounted up to 6.63 % in Montipora. Mean density in Montipora (n = 12) was 3.7 Pedum/ 100 cm-2, mean OCS 2.12 %. From the known dwelling length, the expected volume is found using the potential regression y = 0.2127 x 2.7447.  A high Pedum density indicates a rather near-shore locality with ample suspended nutrients in the water passing by. No indication was found that hosts would suffer seriously from heavy infestation. Corals usually outlive their inhabitants by many years, which is demonstrated by successions of Pedum generations in the same host. The traces, embedded in the skeleton, are not so distinct as in coral associated mytilid Lithophaga (Kleemann 1994). Due to the in comparison to the latter less regular shape of Pedum dwellings and their much wider openings, coral overgrowth results in partly filled and camouflaged dwellings. Nevertheless, they have the potential to yield trace fossils.
 

Bivalve Habitat Expansion of Shoreface Bivalves: A Reconstruction based on the Mesozoic and Cenozoic of Japan

Yasuo Kondo, Koji Hirose, Kazuhiro Sugawara, Naoki Kikuchi, Nobutaka Funayama and Tomoki Hiraoka

Department of Geology, Kochi University, Kochi 780-8520, Japan.
E-mail:  ykondo-u.ac.jp

Habitats of fossil bivalves were reconstructed for more than 50 fossiliferous shoreface sediments spanning from Triassic to Holocene and Recent in Japan, based on sedimentary facies, taphonomical and palaeoecological observations along with examination of published information. The results well outlined long-term history of bivalve habitat expansion to shoreface environment and evolutionary replacement within this environment.  Trigoniids were the chief inhabitants of shoreface in the Mesozoic time.  For example, lower shoreface environments were inhabited by a trigoniid, Vaugonia, in the earliest Jurassic. Nipponitrigonia was probably the first bivalve, which appeared in abundance from upper shoreface sediments, and this occurred in the Late Jurassic or early Cretaceous. Also venerids and glycymeridids occurred in the lower shoreface, but they but did not expand their habitats to the upper shoreface at this time.  Members of the Veneridae and Mactridae successfully established their habitats in the upper shoreface sometime between Late Cretaceous and Miocene. Particularly mactrids became abundant in the Miocene of north Japan, along with other bivalves with various modes of life, including members of the Cardiidae, Tellinidae, Solenidae and Hiatellidae.  The reconstructed colonization history suggests that bivalves expanded their habitats to increasingly more physically unstable, high-energy environments by developing adaptive features, such as (1) large and thick shell (Trigoniidae), (2) streamlined shell form (Veneridae), (3) light-weight shell (Mactridae).
 

Significance of Gill Characters for Taxonomy in Sphaeriidae (Eulamellibranchiata) and Some Other Bivalve Groups

Alexei V. Korniushin

Institute of Zoology, B.Khmelnitsky str.15, 252601-Kiev, Ukraine
Present address: Museum fuer Naturkunde, Invalidenstr. 43, D-10115 Berlin, Germany

Freshwater bivalves of the family Sphaeriidae traditionally arranged in three genera are characterized by a certain reduction of ctenidia. Degree of this reduction varies between genera. In contrast to Sphaerium and Musculium, the outer demibranch of Pisidium (except Pisidium idahoense Roper and P. subtilestriatum Lindholm) consists of the one lamella only.  Specific differences in size and position of the outer demibranch were noticed within the latter genus by earlier investigators.  In this study, position of the outer demibranch was quantified by marking the inner demibranch filament number corresponding to its anterior edge. The obtained figures were treated statistically and differences between species and/or species groups were confirmed. Some patterns of ctenidium ontogenesis were also studied in different sphaeriid taxa. It was shown that position of the outer demibranch usually does not change in ontogenesis;  the time of the outer demibranch appearance and its growth rate significantly varied between species and genera and might be treated as taxonomic characters as well. Correlation between the time of appearance and topographic position of the organ was observed. Noticeable differences in the outer demibranch position and growth rates were also reported for Cerastoderma and Hypanis (Cardioidea) and for Corbicula and Neocorbicula (Corbiculidae). Only the growth rates differed in Unio and Anodonta (Unionidae). One-lamellar ontogenetic stage similar in structure to the gill of Pisidium was observed in Sphaerium, as well as in corbiculids, cardiids and Mya. Other modes of the outer development were observed in mytilids, unionids, dreissenids and scrobiculariids. It was concluded from comparison of these modes, that namely the descending lamella is reduced in the outer demibranch of sphaeriids. The characters observed here were already applied in phylogenetic analysis and taxonomy of Pisidium (on species level). Their applications for the taxonomy of Unionidae, Corbiculidae and Cardiidae on generic level seem to be also rewarding.
 

Physical Constraints in Scallop Swimming: Take-Off and Swimming Mechanics

Michael LaBarbera

Department of Organismal Biology and Anatomy, University of Chicago, 1027 East 57th Street, Chicago, IL 60637, U.S.A.  E-mail:  mlabarbe@midway.uchicago.edu

Take-off from the substrate and swimming were recorded for the scallops Chlamys hastata (N = 10), Chlamys rubida (N = 22), and Crassodoma gigantea (N = 7) using high-speed (125-250 frames/sec) video.  The two Chlamys species swim throughout their lives, but Crassodoma swims only as juveniles; adults are cemented to the substrate. Video recordings were analyzed to determine clap frequency, instantaneous accelerations, and average speed.  Scallops ranged in height from 5-65 mm for the Chlamys species and 26-45 mm for C. gigantea.  Peak acceleration for all three species was approximately 0.5 ms-2; the largest and smallest individuals exhibited lower accelerations than intermediate-sized animals.  Average swimming speed (integrated along the animal’s path) for the three species ranged from 10-40 cm/s.  Larger animals achieved higher absolute swimming speeds, but relative speeds (shell heights/sec) were maximal in the smallest animals and declined linearly with increasing shell size.  For all three species, clap frequency was a linear function of shell height; C. gigantea was indistinguishable from the two Chlamys species during its byssally-attached phase.  Using published data for an additional seven species of scallops, a single function describes the relationship between shell height and clap frequency for all scallops, independent of phylogenetic relationships or environmental temperature.  These data are the first measurements of acceleration during jetting for any scallop and the first quantitative description of swimming in juvenile Crassodoma.
 

Hydrodynamics of Fossil Hippuritids: A Novel Feeding Strategy in an Asiphonate

Michael LaBarbera(1) and Eulàlia Gili(2)

(1) Department of Organismal Biology and Anatomy, University of Chicago, 1027 East 57th St., Chicago, IL 60637, U.S.A.  E-mail:  mlabarbe@midway.uchicago.edu
(2) Dept. de Geologia, Univ. Autònoma de Barcelona, Edifici C, 08193 Bellaterra
(Barcelona), Spain

Hippuritid rudists inhabited Cretaceous shallow carbonate platforms, living partially embedded in the sediment in dense aggregations.  Differential growth of the right valve elevated the commissure above the sediment-water interface.  The operculiform left valve bore a system of radial canals which communicated with the water through pores believed to represent incurrent regions. Hippuritids often preserve inclined 30-45° off vertical, an orientation that appears primary and the result of active growth processes.  Laboratory flume experiments revealed that cylinders tilted downstream generate an intense vortex that lifts water off the substrate to bathe the cap (equivalent to the hippuritid left valve).  Paired model hippuritids were deployed in the Mediterranean Sea in various orientations and configurations; water was drawn through the models and a 2 µm filter in series.  Filters were dried, weighed, and ashed to determine organics captured.  Vertical and